We developed a new measurement system of pupil area with camera position adjustability using iris diameter, which size is constant regardless of automatic nervous system. The camera probe consists of a small CCD camera, infrared LEDs, and a stepping motor. The motor can adjust camera position to an optimum distance in order to get sharp eye images. Pupillary response to cholinergic antagonist was a diagnostic method for Alzheimer's disease. It is very important for diagnosis of dementia to analyze pupil size precisely. We used the new detection system for the pupil test. In this study, we evaluated the auto focus function by detecting model eyes and patients' eyes. The results by using this system show that the proposed system can measure pupil area accurately.